The process is very simple.... and whether you do it at exactly 100 hours is of no concern. Most I see have may time that hours and have never been adjusted. Run synthetic mix and you may never need to (well......).
Remove the 4 back screws, and the three recoil screws. Pull off back and recoil.
Remove the single screw that holds the valve cover on. Rotate the flywheel to TDC on the compression stroke. Insert feeler and adjust the nut on the top of each rocker arm until you can feel friction. Reassemble.
If you have carboned valves from running crappy mix, you need to de-carbon (actually, de-calcify...) with edta solution first, and with great care.... However...100 hours it's not an issue..
You will need the stihl feel gauge - about 50 cents, but it has a specific tiny end that fits into the tappet area.